It is generally agreed that the number of striated skeletal muscle fibers within a muscle remains constant once differentiation of the tissue is complete. However, muscles are able to adapt to a wide variety of physiological situations by altering their myofibril content, i.e., by increasing or decreasing the amount of contractile protein per fiber. The contractile proteins are organized into discrete units or myofibrils which unlike the fibers they comprise are able to grow, divide and increase in number. This research project will investigate the role of functional and developmental parameters (amount of fiber shortening and rate of sarcotubular system growth, respectively) effecting myofibril growth and division in avian skeletal muscle. Electron microscopy will be used to study sarcotubular system growth, fiber size, myofibril size, and myofibril number in fast phasic latissimus dorsi posterior (PLD) and slow tonic latissimus dorsi anterior (ALD) muscles of the chicken. Since the latissimus dorsi anterior and the latissimus dorsi posterior are homogeneous with respect to fiber type (entirely slow tonic fibers and entirely fast phasic fibers respectively) they are particularly well suited for this investigation; any alteration of myofibril fine structure may be related to the particular fiber type. The possible role of sarcomere shortening as a mechanism causing myofibril splitting or division will be evaluated. By examining myofibrils of fibers capable of extensive (PLD) and limited (ALD) sarcotubular development, the role of this tubular system as a mechanism that divides growing myofibrils will also be determined.